The present invention is directed to a method and apparatus for reducing particulates in processing chambers, and more particularly to a method and apparatus for rapidly heating and cooling a vacuum chamber shield.
In substrate processing in general and in physical vapor deposition (PVD) processes such as sputtering in particular, particulates are present and are generated in the processing chamber. These can contaminate and destroy the substrate being processed. When such particulates (also known as “free” particulates) land on the substrate being processed, they can contaminate a small area of the substrate. If the substrate is die cut into separate chips, the contaminated chips can be discarded. However, when a large substrate is intended for subsequent use as a single item, e.g. as a flat panel display, one defect causes the whole unit to be rejected.
The contaminating particulates can originate from several sources. Incomplete or defective chamber cleaning allows particulates to remain in the chamber and cause contamination. Even when the processing chamber is clean, contaminants can be generated during the sputtering process. One type of contaminating particulate originates from sputter deposited material which has deposited itself on processing chamber surfaces other than the substrate intended for deposition. These may subsequentially detach from the location inside the vacuum processing chamber where they originally had been deposited. These particulates are usually cool, multi-molecular sized specks of sputter deposited material which were hot during the sputtering process, but have since cooled as a result of their contact with surrounding surfaces. Such specks can create defects which cause rejection of the substrate.
Another source of particulates is electrical arcing between the highly-negatively charged (biased) target and the surrounding grounded pieces. Arcing occurs in PVD processing chambers at locations between the target and surrounding surfaces, usually a shield enclosing the target. Arcing between adjacent pieces causes a severe localized temperature spike which in most cases releases molecules of one or both of the materials between which the spark arcs. At best, if the released molecules settle on the substrate they create a slight but acceptable anomaly in the coating pattern. At worst, when a particulate is a foreign material, the substrate will be contaminated and will have to be rejected.
In a PVD processing chamber, the target containing the material to be sputtered is generally flat and located parallel to the substrate. Sputtering involves the ionization of gas, e.g. argon (Ar), molecules in the processing chamber. The gas molecules are electrically ionized as a result of an electrical bias, usually a direct current (IC) bias. Once ionized, the positive ions bombard the oppositely-biased target causing the target material to be released into the chamber as molecular size ballistic particles. In the rarified vacuum atmosphere of the vacuum chamber, the target molecules travel nearly unobstructed until they reach the substrate being sputter deposited, which is located just a short distance away.
This sputtering activity coats the substrate as desired by the process, but since the target material being sputtered is emitted in all directions, the surfaces in the processing chamber around the substrate also tend to become coated with sputter-deposited material. These surrounding surfaces are initially generally cold, i.e. at ambient or room temperature. The sputtered material has a very high temperature, usually several thousands of degrees. Upon contact, the sputter-deposited material rapidly cools to the lower temperature of the process chamber surfaces surrounding the substrate. The effect of its condensation on the interior surfaces is to raise the temperature of these interior surfaces to about 180° to 450°.
This may cause some various problems. For example, some of the sputter-deposited material eventually peels off the chamber surface. The tolerances of the components may also be compromised.
To combat these problems, PVD chambers may be constructed with “shield” pieces which act as a lining for the processing chamber. A shadow frame and shield (collectively “shield”) line the inside of the processing chamber substantially between the edge of the target being sputtered and the edge of the substrate. The sputter deposited material then coats the inside of the “shield” and not the inside of the chamber wall. The “shield” can then be easily removed and cleaned or replaced which reduces harmful effects on the chamber wall such as occur if continuously exposed to the ionized process gas.
Arcing around the edge of the sputtering target can also create particulates. Arcing is induced when the biased voltage between the target and a nearby grounded (or dissimilarly biased) member is greater than a certain value. This value is a known function of a multiplicative product of the gas pressure and gap spacing between the target and the nearby grounded (or dissimilarly biased) member. The known numerical relationship is given by Paschen's curves (details of which are considered below). The curves show conditions which are conducive to arcing between the target material and the surrounding shield in the “dark space ring” for a particular gas. An arc jumps between the edges of the biased target and grounded pieces such as the “shield.” The arc causes specks to erupt from the material. Such specks can contaminate the substrate.
The expansion and contraction of process chamber structures due to changes in their temperature affects the gap or clearance between pieces which in turn affect when arcing might occur. One solution to this arcing problem is to maintain the clearance between adjacent pieces (i.e. the shield and the target) at a relatively constant value to prevent arcing. However, it is difficult to keep a constant clearance between the shield and the target material since the shield expands, and its temperature rises due to exposure to ionized gas particles and sputtered material during the process. It is especially difficult to maintain a desired range of clearance dimensions when sputtering is being done for liquid crystal display (LCD) applications. In these applications, the size of the area being sputtered is relatively large (about 470 mm×370 mm), requiring a long and wide shield (outside dimension, e.g., about 660 mm×570 mm) around the perimeter of the target being sputtered. The larger dimensions create large movements due to differential thermal expansion. Further, a slight misalignment or offset of the shield from the target material during assembly of the processing chamber can create a clearance at one side of the chamber which is conducive to arcing, and thus creates particulates. The thermal cycling of the shield elements, which occurs as sputtering is turned on and off, strains the adhesive bond between the sputter-deposited material and the shield pieces. Weakly bonded specks fall or peel off as a result of the thermal cycling.
Another solution to this problem is equalizing the temperature between the shield and the sputter-deposited material by heating the shield to approximately the temperature of the sputter-deposited material. In this way, there is little or no differential thermal expansion between the sputter-deposited material and the shield surface on which it is deposited.
In this solution, the temperature of the heat shield is controlled by an assembly of radiant heaters which are configured to heat the underside of the shield without affecting the chamber process. Heating the shield causes it to expand. The target material also expands so that the actual change in clearance between the edge of the target and the edge of the shield is minimized.
The target material is usually cooled by a liquid such as water to prevent it from overheating. Even though the sputter-deposited material ejected from the target raises the temperature of the surfaces it contacts to about 180° to 450° C., the whole mass of the target material or a target material and backing plate, in those instances where a backing plate is used, has an average temperature of about 50° to 100° C. In this system, a shield having a chairlike or “h” type-shaped cross section is provided with the front of the chair facing the center of the chamber.
The time required to heat and cool a shield of this configuration is on the order of several hours, with the cooling time longer than the heating time. This is partially due to heat transfer from the heaters to the shield, which in the vacuum environment of a processing chamber is by radiation. This is not very efficient. Even venting the chamber with gas does not produce a cooling time of less than two or three hours. Such venting is also inefficient because it depends on the transmission of thermal energy by conduction to the exterior of the hot surfaces. The slow cooling creates a bottleneck in the chamber opening and closing process which detrimentally affects the time that the chamber is available for substrate processing.
These difficulties need to be overcome in order to increase the yield in production of sputtered substrates and reduce or eliminate substrate rejection because of particulate contamination.